The present invention relates to a silent chain drive mechanism for transmitting power between sprockets with different numbers of teeth as achieved when driving camshafts from a crankshaft of a four-stroke engine.
In a four-stroke engine shown in FIG. 6 hereof, a silent chain drive mechanism used for driving camshafts from a crankshaft has two sprockets 11a, 11a on the camshafts side for operating intake valves and exhaust valves, respectively, a sprocket 11b on the crankshaft side, a silent chain S for transmitting power between the crankshaft sprocket 11b and the camshaft sprockets 11a, a tensioner T for applying tension to the silent chain S, a tensioner lever L, and a chain guide G for suppressing run-out of the silent chain S.
The intake and exhaust valves are designed to complete a single cycle of operation while the crankshaft sprocket 11b makes two revolutions. Thus, the rotational speed of the crankshaft sprocket 11b must be reduced to half at the camshafts. To this end, the number of teeth of each of the camshaft sprockets 11a is twice the number of teeth of the crankshaft sprocket 11b. 
The silent chain S used in such silent chain drive mechanism is exemplified in Japanese Patent Laid-open Publication No. Hei-8-184348, which includes, as shown in FIGS. 7 and 8 hereof, a series of longitudinally overlapping link plates 13 (three, 13a, 13b and 13c, being shown) having teeth adapted to mesh with a sprocket (not shown). When the chain is stretched substantially straight, rectilinearly profiled outside flanks 15 of the respective link plates 13a-13c form a series of teeth having a trapezoidal profile.
In one specific form of the exemplified silent chain, when the silent chain is stretched substantially straight as shown in FIG. 7, inside flanks 16 of one link plate 13b are retracted from the outside flanks 15 of the adjacent link plates 13a, 13c. In another specific form of the exemplified silent chain as shown in FIG. 8, when the silent chain is stretched substantially straight, barreled or convexly arcuate inside flanks 16a of one link plate 13b project outward of the outside flanks 15 of the adjacent link plates 13a, 13c. 
The crankshaft sprocket 11b and the camshaft sprockets 11a are formed by a sprocket tooth hobbing process, such as shown in FIG. 9 hereof and described in Japanese Patent Application No. Hei-1-037745. In the hobbing process, a hob cutter with teeth having a profile similar to that of a series of trapezoidal teeth formed by the outside flanks 15 of the longitudinally overlapping link plates 13 described previously is used. The hob cutter cuts the periphery of a cylindrical workpiece with an amount of addendum modification set in such a manner that the silent chain, as it is wrapped around the sprocket, forms an equilateral polygon with one side equal in length to a pitch P of the silent chain. Thus, an involute tooth form is produced.
In the case where sprockets with different numbers of teeth are to be formed to achieve a reduction gear ratio of 1:2, the crankshaft sprocket 11b and the camshaft sprocket 11a are both cut or hobbed by using a hob cutter profiled to have a pressure angle Ah equal to the pressure angle Af defined by the outside flanks of the silent chain. As an alternative, in order to avoid undercut or root thinning caused by the number of teeth of the crankshaft sprocket 11b which is half the number of teeth of the camshaft sprocket 11a, teeth of the crankshaft sprocket 11b are cut by using a hob cutter having a pressure angle larger than the pressure angle Af of the outside flanks 15 of the silent chain.
Thus, the tooth flank configurations of the camshaft sprocket 11a and crankshaft sprocket 11b are formed by involute curves arranged to satisfy the expression
A2xe2x89xa7A1=Af
where A1 is the pressure angle of the camshaft sprocket 11a, A2 is the pressure angle of the crankshaft sprocket 11b, and Af is the pressure angle defined by the outside flanks 15 of the link plates.
A silent chain drive mechanism comprised of the above silent chain S, camshaft sprockets 11a and crankshaft sprocket 11b for driving the camshafts exhibits a meshing condition shown in FIGS. 10 and 11.
First, consider a silent chain S having link plates whose inside flanks are retracted from the outside flanks of the adjacent link plates when the silent chain is stretched substantially straight as shown in FIG. 7. When such silent chain S is wrapped around the sprocket 11 (11a, 11b), the outside flanks 15 of the link plates 13 are seated on the tooth flanks of every other sprocket tooth to assume meshing with the sprocket 11 in a polygonal fashion, while the inside flanks 16 of the link plates 13 do not interfere with the tooth flanks of either of the camshaft sprockets 11a and the crankshaft sprocket 11b because they are retracted from the outside flanks.
However, since the link plates 13 of the above silent chain S mesh at their outside flanks 15 alone with the teeth of each sprocket 11a, 11b, the silent chain S makes, upon commencement of meshing, a polygonal motion involving up and down movements in a radial direction referring to the sprocket.
A quantitative analysis was made on the polygonal motion. As can be appreciated from FIG. 10, since a pitch angle xcex8 of the teeth of the sprocket 11 and the number of teeth Z of the sprocket 11 have relations expressed by xcex8=(360xc2x0/Z), the following may be established.
First, when the chain pitch is P and a free span of the chain following the link plate 13b is positioned at a right angle to a line passing over the centers of the sprocket 11 and a connector pin 12a, as shown in FIG. 10, the distance from the center of the sprocket 11 to the connector pin 12a of the free span chain is determined by the height of the connector pin 12a from the center of sprocket 11 because of the outside flanks of the link plate 13a being seated on the tooth flanks of the sprocket 11, and thus expressed by U=P/2 sin(xcex8/2).
Next, as shown in FIG. 11, when the sprocket 11 is rotated a half pitch angle xcex8/2 from the position of FIG. 10 and the free span chain is positioned at a right angle to a line passing over the centers of the sprocket 11 and the link plate 13b, the distance from the center of the sprocket 11 to the chain pitch line of the free span chain is determined by the height of the connector pin 12a from the center of sprocket 11 because of the outside flanks of the link plate 13a being seated on the tooth flanks of the sprocket, and thus expressed by V=P/2 tan(xcex8/2). In FIGS. 10 and 11, the arrowhead indicates the direction of rotation of the sprocket 11, and the profiled arrow head indicates the direction of travel of the silent chain.
As can be readily appreciated from the above discussion, the silent chain S just started meshing engagement with the sprocket 11 makes a polygonal motion of amplitude Hs=Uxe2x88x92V involving up and down movements during half pitch angle (xcex8/2) rotation of the sprocket 11. Thus, the silent chain S displays a so-called xe2x80x9cchordal actionxe2x80x9d.
Consequently, when the thus-arranged silent chain is used, the chordal action (polygonal motion) of amplitude Hs=Uxe2x88x92V and associated up and down movements cause the outside flanks 15 of the link plates 13a, 13b , 13c to hit the tooth flanks of the sprocket 1, resulting in an undesired beating motion. In addition, when the silent chain drive mechanism is operated at a high speed, undesired vibratory and impact sounds are produced, which form a main source of vibrations and noises generated in the cam drive mechanism of a four-stroke engine.
Furthermore, the chordal action produced by the polygonal motion causes the tension of the chain to change, thereby deteriorating the durability of the silent chain.
Next, consider a silent chain S having link plates whose barreled inside flanks 16a project from the outside flanks 15 of the adjacent link plates when the silent chain is stretched substantially straight as shown in FIG. 8. When such silent chain S is wrapped around the sprocket 11, the outside flanks 15 of the link plates 13 are seated on the tooth flanks of every other sprocket tooth to assume meshing with the sprocket 11 in a polygonal fashion. In this instance, because the barreled inside flanks 16a project at their central portions from the outside flanks 15 of the adjacent link plates, the central portions of the barreled inside flanks 16a of the link plates in a free span chain engage the tooth flanks of the sprocket and temporarily lift up the free span chain before the outside flanks 15 of the link plates are seated on the sprocket tooth flanks, whereby the chordal action can be reduced as compared to the chordal action associated with the silent chain of FIG. 7.
However, since the crankshaft sprocket 11b and the camshaft sprockets 11a have different pitch angles (xcex8=360xc2x0/Z) due to a great difference in the number of teeth, the polygonal motions of the silent chain occurring, respectively, at the crankshaft sprocket 1b and at the camshaft sprockets 1a greatly differ in magnitude from each other. When the above silent chain with link plates having barreled inside flanks 16a is used, the amount of lift of the free span chain is constant, so that the polygonal motion occurs necessarily at either of the two sprocket sides 11a, 11b. Thus, it becomes difficult to suppress both the chordal action occurring at the crankshaft sprocket and the chordal action occurring at the camshaft sprockets.
Accordingly, in such camshaft drive mechanism, even though the chordal action at one of the crankshaft sprocket and the camshaft sprockets can be suppressed, the crankshaft drive mechanism as a whole is still unsatisfactory in reducing vibrations and noises resulting from the chordal action. In addition, there arises a difficult problem that the barreled inside flanks 16a of the link plates hit the tooth flanks of the sprocket, thereby producing another sort of vibration and noise as well as the changes in the chain tension.
It is therefore an object of the present invention to provide a silent chain drive mechanism for transmitting power between sprockets with different numbers of teeth, as achieved when camshafts are driven by a crankshaft of a four-stroke engine via a silent chain, in which the chordal action of the silent chain is suppressed to reduce vibrational sounds and impact sounds caused by the chordal action, and the changes in the chain tension is suppressed, thereby to improve the driving performance and the durability of the silent chain drive mechanism.
To attain the above object, there is provided, in accordance with the present invention, a silent chain drive mechanism which comprises: a larger sprocket having a plurality of spaced teeth; a smaller sprocket having a plurality of spaced teeth smaller in number than the teeth of the larger sprocket; and a silent chain wound around the larger sprocket and the smaller sprocket for transmitting power therebetween, the silent chain comprising a plurality of link plates interconnected by connector pins, each link plate having a pair of teeth, the link plate teeth having inside and outside flanks being profiled to satisfy the expression
Hc=Hf+Hs
where Hc is a distance from a pitch line of the chain to a pitch line of the inside flanks, Hf is a distance from the chain pitch line to a pitch line of the outside flanks, and Hs is an amplitude of a polygonal motion of the chain at a virtual sprocket having a plurality of spaced teeth set to be either equal in number to the teeth of the smaller sprocket or intermediate in number between the teeth of the smaller sprocket and the teeth of the larger sprocket, and the larger and smaller sprockets having respective tooth flanks being profiled by involute curves arranged to satisfy the expression
A1 greater than Acxe2x89xa7A2
where A1 is a pressure angle of the larger sprocket, A2 is a pressure angle of the smaller sprocket, and Ac is a pressure angle of the inside flanks of the link plate teeth of the silent chain.
The silent chain used in the silent chain drive mechanism of the present invention includes a number of intermeshed rows of link plates connected by connector pins such as round pins or rocker joint pins. Each of the link plates has a pair of teeth, and each link plate tooth has an outside flank forming an outer side of the tooth and an inside flank forming an inner side of the tooth.
It is preferable that the virtual sprocket has a pressure angle equal to the pressure angle of the inside flanks of the link plate teeth.
The term xe2x80x9cchain pitch linexe2x80x9d used herein represents a line interconnecting the centers of the connector pins of the link plate.
The term xe2x80x9cinside flank pitch linexe2x80x9d used herein represents a line passing parallel to the chain pitch line over the inside flank at points where the distance between these points becomes xc2xd of the chain pitch.
The term xe2x80x9coutside flank pitch linexe2x80x9d used herein represents a line passing parallel to the chain pitch line over the outside flank at points where the distance between these points becomes {fraction (3/2)} of the chain pitch.
The term xe2x80x9camplitude of polygonal motionxe2x80x9d used herein represents a distance of up and down movements in a radial direction of the sprocket, which the silent chain makes when the chain starts meshing at its outside flanks with the sprocket.
The term xe2x80x9cpressure angle of sprocketxe2x80x9d used herein represents a pressure angle of a tooth form of a hob cutter used to cut the teeth of the sprocket.
In the silent chain drive mechanism of the present invention, since the inside flanks of the link plate teeth are profiled to satisfy the above expression Hc=Hf+Hs, the inside flanks and the sprocket establish therebetween an intermeshing relation similar to that established between a rack and a pinion, in which the distance from the center of the sprocket to a free span of the chain is kept constant throughout the course of meshing of a link plate immediately before the free span chain, that is, from the start of meshing until seating on the sprocket of that link plate, thereby preventing the silent chain from exhibiting a chordal action resulting from the polygonal motion of the chain.
In other words, the chain polygonal motion can be suppressed by using link plates in which the inside flanks are profiled such that the inside flank pitch line is positioned farther from the chain pitch line than the outside flank pitch line by a distance corresponding to the amplitude Hs of the chain polygonal motion occurring at a virtual sprocket having teeth so set as to be either equal in number to the teeth of the smaller sprocket or intermediate in number between the teeth of the smaller sprocket and the teeth of the larger sprocket.
In the silent chain drive mechanism of the present invention, since the respective tooth flank configurations of the larger and smaller sprockets are formed by involute curves arranged to satisfy the above expression A1 greater than Acxe2x89xa7A2, even if the number of teeth of the larger sprocket is twice the number of teeth of the smaller sprocket, the silent chain starts meshing with the larger sprocket and the smaller sprocket such that the inside flank of a link plate immediately before the free span chain meshes with the sprocket concurrently with the outside flank of the preceding link plate being seated on the sprocket to thereby lift up and keep the free span chain at a level corresponding to a peak of the polygonal motion of the chain. The silent chain is thus prevented from exhibiting a chordal action at both the smaller sprocket and the larger sprocket.